How to Cap a Bay Window

Written by melissa smith
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How to Cap a Bay Window
A cap is the roof of a bay window. (upper bay window image by hazel proudlove from

The cap or roof of a bay window works like a hat to keep out moisture from snow and rain and helps to prevent heat loss from outside elements of nature. Over time, strong winds can blow away the shingles or a heavy snow can cause the roof cap to sag and collapse or rain can create a leak that requires the repair or replacement of the roof cap. Replacing or repairing a bay window roof cap requires some basic math skills and heavy lifting, but is not a difficult task.

Skill level:
Moderately Challenging

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Things you need

  • 2-by-4 lumber (for roof cap frame)
  • 1-by-4 lumber (for exterior trim)
  • 3 or more workers for heavy lifting
  • Pneumatic nail gun
  • Fibreglass insulation
  • Exterior grade plywood/decking panels
  • Mitre saw
  • Metal drip cap
  • Roofing felt paper
  • Flashing tape
  • Asphalt shingles
  • Hammer
  • Nails
  • Preset wooden shingles

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    Place and Fill

  1. 1

    Use workers to lift the roof cap into place.

  2. 2

    Nail the cap securely into place.

  3. 3

    Insulate the inner cavity of the roof cap with fibreglass filling.


  1. 1

    Precisely calculate and cut three plywood sheets to cover the roof cap beams. The middle board is rectangle-shaped, while the two side panels are triangle-shaped.

  2. 2

    Use the nail gun to attach the panels to the joists of the roof cap.

  3. 3

    Calculate and bevel-cut the exterior trim so that the ends overlap when connected at different positions along the facade of the roof cap.

  4. 4

    Prevent moisture from seeping in the roof cap by fastening a metal drip cap down the cap's edge.

  5. 5

    Vertically place roofing felt paper on top of the triangle plywood sections. Seal the seam where the felt connects to the house by covering it with a piece of flashing tape.

  6. 6

    Cover the middle rectangle section with roofing felt paper. Make sure the felt edges overlap with the triangle edges. Seal all edges by overlapping with flashing tape.

    Adding Shingles

  1. 1

    Fasten a double layer of asphalt shingles to the bottom edge of the first row only. The first layer should be head to toe, gritty-side up. The second layer lies flat over the first layer, but in the opposite direction with the smoother side up.

  2. 2

    Nail metal flashing at the end of each row, on both sides, where the shingles and the roof cap intersect.

  3. 3

    Start with the third row to stagger the shingles so they don't line up from row to row. When needed, cut the shingles to fit.

  4. 4

    Attach all the shingles, and then insert a long piece of flashing to the rectangle-shaped section.

  5. 5

    Trim and fit the preset wooden shingles. Nail them on top of the flashing to conceal the rough edges of the roof cap.

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